Because of their almost incalculable value to industrial chemical processes, many catalysts and catalytic processes have been developed over the years. Catalysts, such as heterogeneous catalysts, can generally be classified as either supported metal catalysts or massive metal catalysts, also sometimes referred to as bulk metal catalysts.
Supported metal catalysts are usually comprised of relatively small metal crystallites on relatively large support particles. The support particles may also act as a promoter for the active ingredients of the catalyst. Because the metal crystallites are small compared to the support particles, the effective metal surface area per unit volume of catalyst is relatively small, thus limiting the activity of the catalyst.
In contrast, massive metal catalysts, such as the well known magnetite or spinel-based ammonia synthesis catalysts, or Raney metal type catalysts, generally contain metal domains having dimensions which are usually far greater than any promoter phase particles which may be present. The effective metal surface area per unit volume of such catalysts is also relatively small, thus limiting the potential activity of the catalyst. To maximize the effective catalytically active surface area per unit volume, it would be desirable to produce a catalyst having a median metal crystallite, or domain, size of about 25 .ANG. to about 500 .ANG. in diameter. The desired crystallite size, of course, depends on the intended application. For example, crystallite sizes in the range of 25 .ANG. to 250 .ANG. are desired for catalyzing reactions involving relatively small substrate molecules, while larger crystallites, in the range of 250 .ANG. to 500 .ANG., are desired for catalyzing reactions involving large substrate molecules. Effective metal surface area per unit volume of catalyst as used herein means metal area accessible to reactants. Because of the demanding nature of certain catalytic reactions (e.g. ammonia synthesis) the surface area of very small metal crystallites may be either ineffective or non-selective.
Although it may be possible to conventionally produce supported metal catalysts containing metal crystallites having a median diameter within the 25 .ANG. to 500 .ANG. range, it can only be done in combination with relatively large support particles or a relatively large pore support, far in excess of the metal crystallite diameter. Consequently, the effective metal surface area suffers and a catalyst is produced having less activity then it would otherwise have if the size of the support particles were comparable to the size of the metal crystallites.
Consequently, there is a need in the art for the development of catalysts characterized as being balanced with respect to metal surface area and metal crystallite size. That is, catalysts having improved activity owing to their tendency to maximize the effective surface area per unit volume of catalyst.